In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The turbine includes a rotor body with a series of turbine blades extending radially outwardly from the rotor body, and a stationary shroud that forms a tunnel in which the rotor body and its blades turn. The flow of combustion gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a bypass fan that forces air around the center core of the engine, driven by a shaft extending from the turbine section.
The turbine blades are currently made of nickel-base superalloys that have acceptable mechanical properties in the operating conditions of the gas turbine engine. Those nickel-base superalloys are usually coated with a protective coating that protects against oxidation damage. The protective coating includes a non-ceramic coating on the lateral surface of the airfoil. The protective coating may also include a ceramic layer that overlies the non-ceramic coating and insulates the turbine blade to allow it to function for longer times at higher temperatures than would otherwise be possible.
During service and despite the presence of the coating, the tips of some of the turbine blades may be damaged by rubbing contact with the stationary shroud of the gas turbine, by oxidation by the hot combustion gases, and by particle impacts. If the damage becomes sufficiently severe to a turbine-blade tip so that the dimensions of the turbine blade are reduced to less than the specified minimum values and/or the overall engine performance becomes unacceptable, the damaged turbine blade is removed from service. The damaged turbine blade may then be repaired and returned to service or discarded, but repair is preferred because of the high cost of each new turbine blade. The decision to repair or discard is in part economic, so that the higher the cost of the repair, the less likely that the turbine blade will be repaired and the more likely that an expensive new turbine blade will be installed.
In the conventional repair process as now practiced, adjacent protective coatings are removed, the damaged material at the tip is removed, repair material is applied to restore the dimensions of the turbine blade to the specified range, the lateral surface of the tip area is recoated, and the repaired-and-recoated turbine blade is heat treated. To improve overall gas-turbine-engine economics, there is a need to reduce the cost of the repair. The present invention fulfills this need, and further provides related advantages.